Various printing devices have been developed for a variety of applications, including both large scale printing on banners and other signage items as well as small scale general consumer printing. These printing devices typically include a number of printheads. Each printhead includes a number of printhead modules. A printhead module includes a set of nozzles configured to eject a marking fluid, such as ink, onto a print medium, such as paper.
The printheads are often secured to a movable platform referred to as a carriage. As the carriage moves in relation to a print medium, a control system individually controls each printhead module to selectively eject marking fluid to create a desired image on the print medium.
A common method of ejecting marking fluid from the printhead modules is to place a piezoelectric or other type of actuator within each nozzle. A piezoelectric actuator is a material that expands when a voltage is applied. This voltage is referred to as the drive voltage. With the proper drive voltage being applied to the actuator, the expansion of the actuator causes the marking fluid within the nozzle to be ejected onto the print medium. In general, a higher drive voltage causes a greater volume of marking fluid to be ejected from the nozzle. In other printing devices, a different type of actuator may be used. For example, in a thermal inkjet printing system, a heating element, such as a resistor, is used to rapidly heat the marking fluid in the nozzle. The thermal energy causes marking fluid to be ejected from the nozzle onto the print medium.
Printing devices that perform a high volume of print jobs experience mechanical degradation of their actuators over time. As a nozzle actuator expands and contracts repeatedly over the life of the printing device, the actuator degrades such that the volume of marking fluid which is ejected from the nozzle changes. This change in volume of marking fluid ejected affects the optical density of the image being printed onto the print medium. Optical density refers to the quantity of marking fluid used, per unit area, to form the desired image.
Furthermore, different printhead modules often degrade at different rates based on different levels of usage and minor variations that may naturally occur during manufacturing. This creates inconsistencies in the optical density of portions of an image being printed by different printhead modules within the printhead system. These inconsistencies in optical density adversely affect the quality of the image being printed.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.